Method for making niobium nitride fibers

ABSTRACT

1. A METHOD FOR MAKAING A NIOBIUM FIBER COMPRISING: (A) HEATING AN ALLOY CONTAINING NICKEL AND NIOBIUM TO ITS MELTING POINT; (B) IMMERSING A BORON NITRIDE FIBER IN THE MOLTEN ALLOY, WHEREIN THE BORON IS REPLACED BY NIOBIUM TO FORM THE NIOBIUM NITRIDE FIBER; AND (C) WITHDRAWING THE NIOBIUM NITRIDE FIBER FROM THE MOLTEN ALLOY.

Oct. 1,1974

P. T. 'B. SHAFFER METHOD FOR MAKING NIOBLUM NJIITRIDE FIBERS Filed 001;. 16, 1972 United States Patent 3,839,544 METHOD FOR MAKING NIOBIUM NITRIDE FIBERS Peter T. B. Shaffer, Grand Island, N.Y., assignor to The Carborundum Company, Niagara Falls, N.Y. Filed Oct. 16, 1972, Ser. No. 298,072 Int. Cl. C01b 21/06 U.S. Cl. 423-406 3 Claims ABSTRACT OF THE DISCLOSURE A method for converting precursor nitride fibers into niobium nitride fibers by immersing the fibers in a molten bath of a niobium alloy in the temperature range of 1200 to 1400 C. Unreacted alloy is subsequently removed from the fibers by immersion in an acid bath, the fibers then being washed and dried. Niobium nitride fibers are useful in the formation of superconducting devices and may also be used as filtration media or as an insulation for high temperature applications.

BACKGROUND OF THE INVENTION Until a relatively few years ago, fibers of nitride materials were unknown, although the bulk nitrides of many metals and non-metals have been known for a long time. Among the first successful nitride fibers is that described in U.S. Pat. 3,429,722 issued to James Economy et al., in which the preparation and properties of boron nitride fibers is disclosed. In US. Pat. 3,380,935, issued to H. F. Ring, the preparation of metallic nitride fibers of metals such as niobium and titanium are disclosed. Both of these nitrides exhibit super conducting properties in their bulk state and these properties were increased when the nitrides were made in fibrous form. Of these fibers, niobium nitride appears to have the best superconducting properties. A general method of preparation for metallic nitrides in fibrous form involves the reduction of one of the metal chlorides in a nitrogen atmosphere, using a reducing agent such as hydrogen. Niobium nitride fibers, prepared by this procedure, may range in fiber diameter from .01 micron to 20 microns and have lengths of from 0.1 millimeters to 1 centimeter. The procedure is not entirely satisfactory, since crystalline deposits may form on the fibers during their deposition while the size of the crystals within the fiber itself is difiicult to control. Inter-fiber bonding may also occur which may then cause surface imperfections when the fibers are combined into a filament. A more satisfactory method, using a precursor fiber of controlled structure which could be in turn converted into niobium nitride of controlled crystal structure and fiber strength is therefore desirable, especially since the superconducting properties of the fiber appear to be directly related to fiber diameter, a variable which is not directly controllable by the methods of fiber production as described.

SUMMARY OF THE INVENTION A method for making a niobium nitride fiber which comprises immersing a fiber of boron nitride in a bath of bulk niobium nickel alloy, wherein the boron is replaced by niobium to form the niobium nitride fiber and then withdrawing the fiber and immersing it in a bath of acid to remove unreacted alloy. The crystal structure of the resulting niobium nitride fiber and therefore the degree of inter-crystalline bonding may be controlled by the temperature and composition of the alloy bath, as well as by the diameter of the boron nitride precursor fiber and the duration of its immersion in the alloy. Stranded filaments are readily processed by this method, the molten alloy bath assuring uniform fiber content and minimizing undesirable inter-fiber bonding.

3,839,544 Patented Oct. 1, 1974 ice 2 BRIEF DESCRIPTION OFTHE DRAWING The drawing shows a diagram of the method for drawing precursor boron nitride fibers through an alloy bath to form niobium nitride fibers.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of the invention comprises making a niobium nitride fiber by drawing a boron nitride fiber through a bath of a molten alloy containing niobium. As the fiber passes through the bath the boron is replaced by niobium to give the niobium nitride fiber. The composition of the alloy in the bath and the duration of fiber exposure may be controlled to give a fiber consisting essentially of niobium nitride. The fiber is then drawn through an acid bath to remove excess alloy which may be physically held on the fiber.

The apparatus used in this method is shown in the drawing. Since the alloy bath must be heated at temperatures ranging from about 1200 to about 1400 C. the container holding the bath must be a suitable refractory material which is non-reactive with the metal alloy. A preferred refractory material for this purpose is bulk niobium nitride. This has a melting point of over 2500 C. and does not react with the niobium alloy employed in the invention. This refractory container is shown at 10 in the drawing, the container being enclosed by one or more layers of insulation 12. While other heating means may be employed, the alloy bath 14 is preferably melted by induction heating, the induction coil being shown at 16. The alloy bath is protected from the atmosphere by a refractory insulating cover 18 which contains openings at 20 and 20a to allow passage of the fiber to and from the bath. A tube 22 is provided in cover 18 through which an inert gas such as nitrogen or argon may be supplied to maintain a non-oxidizing atmosphere above the alloy bath. During operation, a boron nitride fiber or filament 24 is drawn from a supply reel 26 and over guide wheel 28, from which the fiber then passes through one of the openings 20 in the cover 18 and is immersed in the molten alloy bath 14 by the action of a submerged refractory semicircular guide 30. The rate of fiber movement is controlled so that conversion to niobium nitride is essentially complete by the time the fiber has completed its movement through the alloy bath. The converted fiber then leaves the bath through opening 20a and cover 18, passing over guide wheel 32. From guide wheel 32 the niobium nitride fiber 34 then passes over guide wheel 36 and into an acid bath 38, contained in an appropriate vessel 40. The composition of the acid bath is not critical; nitric acid is preferred, but any mineral acid or mixture of acid may be used which will dissolve the traces of unreactive alloy from the niobium nitride fiber but will not attack the fiber itself. The acid bath has a cover 42 which contains openings 44 for the entrance and exit of the fiber. The cover 42 also supports a semi-circular fiber guide 46 which acts to immerse the fiber below the surface of the acid bath 38. The fiber is withdrawn from the acid bath and passes over guide wheel 48, from which it may then pass through a water washing step 50, and drying 52, prior to collection on reel 54. The water washing and drying steps are conventional ones and are therefore merely indicated on the drawing.

The boron nitride precursor fiber used in the method of the invention may consist of boron nitride fibers with diameters ranging from 10 to 20 microns combined into a continuous strand which may have a diameter ranging from one tenth to one millimeter. The diameters of the individual nitride fibers have a marked affect on the time required for the replacement reaction with niobium, since this depends on the diffusion rate of the niobium into the fiber as well as the diffusion rate of the displaced boron. The desired replacement is therefore favored by fibers having diameters of microns or less.

Relatively high temperatures are required if the replacement reaction is to take place quickly enough for the method of the invention to be practical. This temperature may therefore range from about 1000" to above 1500 C. a preferred range being about 1200 to 1400 C. Since pure niobium has a melting point of over 2400 C., the use of a lower melting alloy is essential to the success of the method of the invention. While an alloy of about 30 weight percent niobium and 70 percent thorium (melting point approximately 1450 C.) may be used, the preferred alloy is one of niobium and nickel in which the niobium content may range from 25 to 60 weight percent with nickel ranging from 40 to 75 percent, while the melting point may vary from about 1200 to 1400 C. The use of the niobium nickel alloy therefore provides a range of niobium concentrations and melting points which may be varied to provide the optimum reaction rate for the conversion of boron nitride fiber to niobium nitride. The use of a non-oxidizing gas over the surface of the molten alloy prevents oxidation, both of the alloy and the filament entering and leaving the hot alloy bath.

After leaving the alloy bath, the niobium nitride fiber will retain small amounts of unreacted alloy on the surfaces of the fibers; this is removed by subsequent immersion in a solution of a mineral acid, such as nitric, sulfuric or hydrochloric acid which will react with the alloy. Obviously an acid or mixture of acids which reacts with the niobium nitride fiber itself cannot be used. Of the acids mentioned, nitric acid is preferred. The concentration of the acid in the solution is not critical to the invention and may be adjusted as required for complete removal of the excess alloy from the fiber.

At sufiiciently low temperatures, niobium is a superconductor and this behavior is likewise shown by the nitride. In bulk form, the nitride shows superconductivity up to a transition point of about K., while the fibrous nitride may have a transition point of 17 K. Fiber diameter is a critical dimension since critical current values may increase rapidly as fiber diameter decreases below a diameter of about 20 microns. Since the method of the invention may begin with precursor fibers of diameters within a desired range, the resulting niobium nitride fibers will retain these diameters which exhibit superconducting properties. Fiber strength is an important factor, this in turn depending on the crystalline structure and the intercrystalline bonding within the fiber. The crystal structure is difficult to control when fibers are made by the usual methods of vapor deposition. When the fiber is formed by a replacement reaction, however, as in the method of the invention, crystalline structure may be controlled by variations in the composition of the niobium alloy bath, the temperature of the bath, and the time of immersion of the fiber. The undesirable bonding between fibers within a strand is also minimized since the replacement reaction does not build up additional material on the fiber surface but instead reacts within the fiber.

Superconducting fibers of niobium nitride are useful in the fabrication of circuit components for low frequency and high frequency devices, radiation detectors, computer memory devices and the like. The fibers might also provide the superconducting pathway for a composite wire in which the fibers would comprise a core surrounded by a jacket material of some ductile material. The high current carrying capabilities of this wire would make it useful for making superconducting magnets, transformers and similar electrical devices. Apart from their superconducting properties, niobium nitride fibers show extremely good thermal stability, inertness, and strength. Fibrous mats may be made which are useful as filters and as thermal insulation for high temperature applications. The fibers may be incorporated into plastics to increase stiffness and tear strength or may be used as reinforcing agents for paper or similar fibrous products.

What is claimed is:

1. A method for making a niobium fiber comprising;

(a) heating an alloy containing nickel and niobium to its melting point;

(b) immersing a boron nitride fiber in the molten alloy, whereby the boron is replaced by niobium to form the niobium nitride fiber; and

(c) withdrawing the niobium nitride fiber from the molten alloy.

2. The method according to claim 1 wherein the alloy comprises niobium and nickel with the niobium content ranging from about 25 to weight percent and the nickel content ranging from about to about 40 weight percent.

3. The method according to claim 1 wherein the alloy is heated in a range from about 1200 to about 1400 C References Cited UNITED STATES PATENTS OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner 

1. A METHOD FOR MAKAING A NIOBIUM FIBER COMPRISING: (A) HEATING AN ALLOY CONTAINING NICKEL AND NIOBIUM TO ITS MELTING POINT; (B) IMMERSING A BORON NITRIDE FIBER IN THE MOLTEN ALLOY, WHEREIN THE BORON IS REPLACED BY NIOBIUM TO FORM THE NIOBIUM NITRIDE FIBER; AND (C) WITHDRAWING THE NIOBIUM NITRIDE FIBER FROM THE MOLTEN ALLOY. 